I have a Dell mini with version 8.04. I can not figure out how to upgrade it to any version of Ubuntu I have bought. I can not even boot from a disc. The upgrade manager tries, but always fails. My system works great. You might say if it works, leave it alone, but still why not?
I'm assuming you have the Dell Mini 9 which came pre-loaded with 8.04 from Dell which was a customized image to fit the small SSD.
There is a very good answer on Ubuntu Forums that should help you fix the issue but to summarize;